1. Field of the Invention
The present invention relates to a charging device for uniformly charging the surface of a photoconductive body, i.e., a charge acceptor, and an electrophotographic image forming apparatus provided with this charging device and, more particularly, a charging device to be disposed with its charging electrode in contact with the surface of a photoconductive body, and an electrophotographic image forming apparatus provided with this charging device.
2. Description of the Related Art
An image forming apparatus, such as a copying machine or a printer, charges the surface of a photoconductive body, i.e., a charge acceptor, by a charging device, forms an electrostatic latent image on the surface of the photoconductive body by irradiating the surface of the photoconductive body with imaging light, and develops the electrostatic latent image in a visible image by applying a developer to the electrostatic latent image. Some conventional charging devices of a corona charging system for use on such an electrophotographic image forming apparatus use corona discharge and others of a contact charging system use a charging roller or the like.
A charging device of a corona discharge system has a discharge wire extended near the surface of a charge acceptor in a shield case, applies a high voltage across the discharge wire to initiate a corona discharge to charge the surface of the charge acceptor with a predetermined charge. Although this known charging device exerts an excellent performance in uniformly charging the charge acceptor, the discharge device produces a large amount of discharge products, such as ozone, needs to remove the discharge products, which, in most cases, increases the size and the cost of the discharge device.
Accordingly, a discharge device of a contact charging system provided with a charging electrode to be disposed in direct contact with a charge acceptor has been used. The charging device of a contact charging system has a conductive charging member, such as an elastic, conductive charging roller or a conductive charging brush, disposed in contact with the surface of a charge acceptor, and applies a charging voltage across the conductive charging member to pass an electrical discharge across a minute gap near the position where the conductive charging member is in contact with the surface of the charge acceptor to charge the charge acceptor. Charging devices disclosed in Japanese Patent Laid-open Nos. Hei 1-93760 and Hei 3-203754 employ a blade-like charging electrode pressed against the surface of a charge acceptor to use the charging electrode also as a cleaning blade for removing residual toner from the surface of the charge acceptor. A charging device disclosed in Japanese Patent Laid-open No. Hei 4-249270 employs a flexible film as a charging electrode, disposes the flexible film with its working edge in contact with the surface of a charge acceptor.
These known charging devices produce only a very small amount of ozone because they do not use corona discharge, and can be formed in a small, lightweight construction because the conductive charging member is disposed in contact with the charge acceptor.
The charging device of a contact charging system employing a conductive charging roller needs a supporting mechanism for supporting the charging roller, and hence, in most cases, has a complex construction. The elastic charging roller must be in close contact with the charge acceptor so that a stable minute gap is formed to charge the charge acceptor uniformly, and hence, the hardness of a rubber coating forming the surface of the elastic charging roller must be comparatively low. Such a rubber coating contains a comparatively large amount of process oil and hence the process oil is liable to be transferred to the charge acceptor to affect image quality adversely. A means for overcoming such a disadvantage of the charging roller finishes the charging roller in a higher dimensional accuracy. However, it is very difficult to finish a charging roller provided with a rubber coating in a high dimensional accuracy, so that the yield of the charging roller manufacturing process is reduced entailing increase in the cost.
The charging device employing a conductive brush is advantageous, as compared with the charging device employing the charging roller, in placing the charging member in uniform contact with the charge acceptor because the conductive brush can be easily disposed so as to be in uniform contact with the charge acceptor. However, the charging brush needs much time and labor to manufacture and is liable to form brush marks that cause irregular charging adversely affecting the image.
The blade-like charging member which is used also as a cleaning blade is difficult to dispose properly so that the charging member is able to remove the residual toner satisfactorily and to form a minute gap necessary for discharge accurately. Therefore, it is difficult to charge the charge acceptor uniformly and satisfactorily by using the blade-like charging member.
The film-shaped charging electrode, as compared with other conductive charging members, is simple in construction, is easy to dispose in stable contact with the charge acceptor and can be manufactured at a low manufacturing cost. However, the film-shaped charging electrode vibrates due to frictional electrification because the working edge of the film-shaped charging electrode is in contact with the charge acceptor, whereby the charging potential is liable to be caused to become unstable. Furthermore, if foreign matters, such as toner and additives, adhere to the working edge of the film-like charging electrode, creeping discharge occurs to cause defective stripes of charges. A method to solve such a problem applies both a DC voltage and an AC voltage simultaneously to the film-shaped charging electrode. However, the AC voltage generates vibrations resonant with the frequency of the AC voltage and generates charging noise.
A charging device proposed in Japanese Patent Laid-open No. Hei 4-232977 or Hei 5-72869 to eliminate those disadvantages employs a cylindrical charging electrode formed by rolling a flexible film in a cylindrical shape, and supports the cylindrical charging electrode in contact with the circumference of a support roller so that the cylindrical charging electrode is in contact with a charge acceptor in a partly collapsed shape. The circumference of the charging electrode moves like an endless belt in the circumferential direction of movement of the charge acceptor as the support roller is driven for rotation. The charging electrode may be pressed against the circumference of the support roller to keep the charging roller in steady contact with the support roller. Since the charging electrode rotates at the same surface speed equal to that of the charge acceptor, the charging electrode and the charge acceptor are not frictionally electrified, faulty charging can be prevented, foreign matters, such as toner and the like, adhere scarcely to the charging electrode, irregularity in charge distribution on the circumference of the charge acceptor is reduced and, even if foreign matters adhere to the charging electrode, the formation of stripe-shaped defects in the image can be avoided because the charging electrode rotates.
This charging electrode, however, must be formed of a film of a small thickness, as compared with that of the film employed in the charging device disclosed in Japanese Patent Laid-open No. Hei 4-249270, to secure uniform contact between the charging electrode and the charge acceptor. Therefore, the charging electrode is liable to be deformed and, consequently, the charging electrode is strained by an excessively large force acting on a portion of the charging electrode in contact with the support roller, and the charging roller is wrinkled, dislocated and twisted in a long period of use.
When the charging electrode is pressed in close contact with the circumference of the support roller by a pressing member, a frictional force exerted by the pressing member on the charging electrode distorts the charging electrode, which is liable to form an irregular distribution of contact pressure between the charging electrode and the charge acceptor. Consequently, a uniform potential distribution of charges cannot be stably maintained. Furthermore, it is difficult to drive the support roller and the charge acceptor synchronously at the same surface speed and hence the frictional electrification of the charging electrode and the charge acceptor cannot be perfectly prevented.